Seventy-one strains of viridans streptococci, classified as Streptococcus sanguis, S. gordonii, S. oralis, S. mitis or S. anginosus, by a revised taxonomic scheme, were characterized and compared by their specific adherence related properties. Significant differences were noted between species. Strains of S. sanguis, S. gordonii and S. oralis commonly expressed neuraminidase sensitive hemagglutinating activities while aggregation of acidic proline rich protein coated latex beads was noted primarily with strains of the latter two species. More strains of S. sanguis, S. gordonii and S. oralis were specifically adherent to saliva-coated hydroxyapatite than were strains of S. mitis and S. anginosus. Lactose resistant coaggregations with actinomyces were noted most frequently with S. gordonii and S. anginosus, species not involved in the primary colonization of teeth. In contrast, lactose sensitive coaggregations with actinomyces were observed with all strains of S. oralis and less frequently with the other streptococcal species. Many of these streptococci also participated in GalNAc sensitive coaagregations with certain S. sanguis and S. gordonii strains. Actinomyces with Gal and GalNAc sensitive lectins and streptococci with GalNAc sensitive lectins both coaggregated with S. oralis 34 but these bacteria did not coaggregate with a spontaneous mutant of strain 34 that lacked a cell wall polysaccharide previously shown to contain GalNAcbeta13Ga1alpha receptor regions. Other strains known to possess cell wall polysaccharides containing this disaccharide also coaggregated with streptococci expressing GalNAc sensitive lectins. Unlike actinomyces lectins, the streptococcal lectins failed to recognize Galbeta13GalNAcalpha in polysaccharide receptors of other streptococci. The association of certain adherence properties with different taxonomic groups of viridans streptococci and definition of the specificities of some of these interactions provide a basis for further ecological characterization of these oral bacteria.